Hydraulic flow distribution system for multiple pass air cooled heat exchanger

ABSTRACT

A multiple pass heat exchanger for cooling a fluid and having upright fluid conveying structure including separate, intercommunicating heat exchanger tube sections for upward and downward flow of fluid relative to a cooling medium such as air and wherein a fluid supply header is joined to the lower end of one of the sections, an overflow weir box is provided at the lower end of the other section with the weir box having an overflow opening while the header has an inlet opening, a fluid supply line is joined to the header inlet and has a valve controlled by-pass line extending therefrom and the weir box and header both have drain orifices therein of smaller size than the overflow opening and inlet respectively, yet larger in diameter than the inlets of the heat exchanger tubes whereby upon interruption of the normal supply of fluid to the sections through said supply line, automatic draining of the sections occurs without the use of mechanical or electrical controls. Similarly, automatic filling of the sections may be accomplished, and if desired any proportion of the fluid to be cooled may be by-passed around the sections.

United States Patent 1191 Cates [111 3,782,451 1 Jan. 1,1974

[ HYDRAULIC FLOW DISTRIBUTION SYSTEM FOR MULTIPLE PASS AIR COOLED HEAT EXCHANGER [75] Inventor: Robert E. Cates, Leawood, Kans.

[73] Assignee: The Marley Company, Mission,

Kans.

22 Filed: June 19, 1972 211 App]. No.: 264,150

Primary ExaminerAlbert W Davis, Jr. Assistant ExaminerS. J. Richter An0rneyGordon D. Schmidt et a1.

flmb. Air

[57] ABSTRACT A multiple pass heat exchanger for cooling a fluid and having upright fluid conveying structure including sep arate, intercommunicating heat exchanger tube sections for upward and downward flow of fluid relative to a cooling medium such as air and wherein a fluid supply header is joined to the lower end of one of the sections, an overflow weir box is provided at the lower end of the other section with the weir box having an overflow opening while the header has an inlet opening, a fluid supply line is joined to the header inlet and has a valve controlled by-pass line extending therefrom and the weir box and header both have drain orifices therein of smaller size than the overflow opening and inlet respectively, yet larger in diameter than the inlets of the heat exchanger tubes whereby upon interruption of the normal supply of fluid to the sections through said supply line, automatic draining of the sections occurs without the use of mechanical or electrical controls. Similarly, automatic filling of the sections may be accomplished, and if desired any proportion of the fluid to be cooled may be by-passed around the sections.

17 Claims, 6 Drawing Figures Heated Air PATENTEUJAH 1mm 3,782,451

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SHEET 2 BF 2 flmb. Air

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H01 Wafer HYDRAULIC FLOW DISTRIBUTION SYSTEM FOR MULTIPLE PASS AIR COOLED HEAT EXCHANGER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to heat exchange structure especially useful for cooling a fluid, as for example water, by bringing the hot liquid into thermal interchange relationship with a cooling medium such as air and wherein a unique filling and draining system is incorporated into the apparatus for providing automatic venting, draining and filling of the exchanger without the necessity of employing mechanical or electrical controls for start-up, normal operation, or shut-down.

2. Description of Prior Structures Although the hydraulic flow-segretation system for air cooled heat exchange equipment embodying the novel concepts of this invention has utility for many applications, it is particularly advantageous wnen incorporated into a water cooling tower of the type wherein the hot water is initially directed through a heat exchange section wherein the water is brought into indirect thermal interchange relationship with cooling, air, and is then discharged to a hot water distribution basin. Water which collects in the distribution basin is divided into a large number of individual streams for gravitation onto the cooling tower fill assembly for evaporative cooling of the water prior to its collection in a cold water basin therebelow. Cooling air streams from the ambient atmosphere passing through the dry section and wet section in parallel are generally combined prior to discharge from the tower. A stack for inducing natural draft flow of air through the dry heat exchanger and the evaporative cooling section comprises one form of structure for drawing adequate quantities of cooling air across the dry and wet heat exchange sections, or motor driven fan means may be used with equal utility depending upon the type of tower required for a particular job specification and the amount of liquid which must be handled per unit of time.

Specifically, air cooled. heat exchange structures for cooling liquids such as water have heretofore presented a number of problems which were difficult to solve not only because of the necessity of preventing freeze-up of the fluid containing tubes in cases of forced or selective shut-down of the water supply pump, but also because of start-up difficulties attributable to the tendency for the tubes to fill unequally and thereby produce uneven flow of water to be cooled through the tube sections. Other heretofore unresolved operating problems included difficulties encountered with blockage of the finned tubes with extraneous solid materials and similar deposits. Prior efforts to provide solutions to the problems detailed above have proven to be relatively expensive and oftentimes unreliable primarily because of the complicated nature of the. equipment necessary to provide effective flow control for normal operation of the heat exchanger, as well as during start-up filling and necessary draining of the. system when the fluid supply is either deliberately or inadvertently interrupted. One significant difficulty in this respect is the fact that equipment of this type is normally operated in unattended manner except for occasional. maintenance or inspection of the operating characteristics of the. apparatus, and therefore, damage often occurred upon shutdown of the hot water supply by virtue of the fact that in the case of cold weather operation, the tubes could freeze up before an operator was actually aware of difficulty in that particular heat exchange section. Furthermore, mechanical and electrical controls, particularly where valves are involved, present operation problems which can affect reliability in cold weather use at subfreezing temperatures.

SUMMARY OF THE INVENTION It is therefore the primary object of the present invention to provide a hydraulic flow-segregation system for heat exchange apparatus operable to cool a fluid by bringing the same into indirect heat exchange relationship with a cooling medium wherein automatic venting, draining and filling of liquid is provided without use of mechanical or electrical controls and which is capable of assuring avoidance of freeze-up of the heat exchanger tubes when supply of hot liquid to be cooled ceases, yet assures uniform and rapid filling of the system with hot water during start-up without starving of portions of the heat exchanger tubes even though a portion of the heat exchangers may be operating in a partial vacuum such as occurs in the loop of a siphonpiping system, and also is operable in a manner such that the user may regulate-the flow of fluid through the heat exchanger as desired without affecting the inherent rapid draining and filling characteristics of the equipment.

A further object of the invention is to provide hydraulic flow-segregation apparatus as described which embodies a common vent manifold connecting the topmost point of parallel exchangers which are piped in a siphon loop of a piping system for the purpose of preventing the untimely initiation of siphon flow in individual exchangers. Thus the system permits siphon establishment only after the liquid to be cooled is made available to the top most portions of all individual exchangers.

A still further important object of the invention is to provide hydraulic flow-segregation apparatus for a heat exchanger which is especially applicable to multiple pass heat exchangers but may be used on single pass exchangers as well, without clogging of the exchanger tubes occurring by virtue of accumlation of solid foreign materials in the tubes, and also may be properly sized for use in any type of cooling tower regardless of the capacity thereof or the means employed to effect flow of cooling air across the tube sections.

A still further important object of the invention is to provide a hydraulic flow-segregation system for a heat exchanger which is adapted to bring the fluid to be cooled into indirect thermal interchange relationship with a cooling medium wherein the flow controls may be calibrated during initial start-up with manually positionable valves whereby the system may be started and stoppedv periodically as necessary or desired with freeze-up of the heat exchanger under cold operation conditions being avoided by virtue of automatic draining of the heat exchange tubes upon interruption of supply of fluid thereto.

A further important object of the invention is to provide a hydraulic flow-segregation system for controlling flow of fluid through a heat exchanger of the indirect thermal interchange type which is not only useable with single as well as multiple pass tubes, but also particularly useful in the case where it is desired that at least a part of the liquid supply be by-passed around the heat exchanger for the purpose of accommodating less water flow through the dry sections than through the wet section to maintain a minimum head loss to the pumping system while providing adequate flow to the dry heat exchangers. The system is also useable for various applications regardless of the direction of water supply relative to the heat exchange tube sections for maximum space saving and equipment orientation convenience.

Other objects of the present invention will be explained or become apparent as the following specification progresses.

DESCRIPTION OF DRAWINGS In the drawings:

FIG. I is an essentially schematic representation of an induced draft water cooling tower of the type having a dry cooling section as well as an evaporative segment and illustrating hydraulic flow-segregation apparatus for the dry exchanger section which provides automatic venting, draining and filling of liquid under the control of manually positionable valves.

FIG. 2 is a fragmentary, enlarged, vertical crosssectional view of the schematic showing of FIG. 1 and more specifically illustrating the multiple pass heat exchanger structure, inboard hot water supply means and the components for effecting automatic venting, draining and filling of liquid responsive to operation or discontinuance of the fluid supply pump;

FIG. 3 is a schematic, fragmentary, enlarged crosssectional view of multiple pass heat exchange structure incorporating automatic venting, draining and filling of liquid similar to the structure of FIG. 2, but illustrating the way in which the liquid to be cooled may be supplied to the heat exchanger outboard from the side thereof illustrated in FIG. 2;

FIG. 4 is also an enlarged, schematic, fragmentary vertical cross-sectional view of hydraulic flowsegregation apparatus similar to that previously described but in this case having a single flow path for the liquid relative to the air flow (noting in this respect that the upright supply riser is illustrated as being out of the plane of the heat exchange tubes for clarity, but it is to be understood that in most instances, the upright supply riser is to be located between adjacent upright tube sections of the heat exchanger);

FIG. 5 is a fragmentary, enlarged, schematic, generally vertical cross-sectional view of the exchanger apparatus illustrated in FIG. 2 and showing the way in which start up of the heat exchanger may be accomplished in a manner such that the sections are all filled with liquid while starving of adjacent exchangers is avoided; and

FIG. 6 is a schematic, fragmentary, enlarged vertical cross-sectional view of the same structure illustrated in FIG. 5 but showing the way in which automatic draining of the heat exchanger tubes occurs upon interruption of supply of liquid to the apparatus.

DESCRIPTION OF PREFERRED EMBODIMENTS The cooling tower 10 shown in schematic form in FIG. 1 of the drawings has been illustrated as being of the induced draft type having a motor driven fan 12 for pulling ambient air through opposed inlet louver faces 14 of the tower casing for passage through the dry and wet sections 16 and 18 thereof respectively. The mixture 0f we. and dry heated air is ultimately discharged through the air velocity increasing stack 19. In fact though, the novel concepts of this invention are equally applicable to a natural draft tower which usually has a hyperbolic stack located with the dry and wet sections 16 and 18 around the perimeter of the base of the stack. Thus, the mechanical draft tower 10illustrated in FIG. 1 has been shown therein only for purposes of simplification and is not to be deemed a limitation of the scope of this invention.

The wet section 18 of tower 10 is of generally conventional construction and operation. For example, the open topped hot water distribution basin 20 each have a series of orifices therein for delivering individual streams of hot water from basin 20 onto the upper horizontal face of a corresponding fill assembly 22. Air drawn into the interior of the casing of tower 10 through each inlet face is brought into cross flow relationship with water gravitating from a respective basin 20 into the cold water collection basin 24 underlying the fill assemblies 22.

Hot water to be cooled (typically water from the condensing coils of an electrical power generating plant or the like) is furnished via supply line 26 leading to risers 28 in turn connected to respective horizontal manifolds 29. Header feed lines 31 extend from corresponding manifolds 29 across hot water distribution basins 20. Since the air cooled heat exchanger structures 30 are intended as being of essentially identical construction, only one of the same will be described in detail. Thus, as is evident from FIG. 2, the multiple pass dry heat exchanger 30 includes a number of upright, finned heat exchange tubes 32 which extend from hot water header 34 to the reverse-flow manifold header 36 communicating with tubes 32 at the uppermost ends thereof. A plurality of other finned heat exchange tubes 37 extend downwardly from header 36 and terminate within the interior of weir box 38. As is clearest from FIG. 2, weir box 38 is open at the top thereof presenting an overflow opening 40 therein and has a drain orifice 42 in the bottom thereof. Similarly, header 34 has a drain orifice 44 and it is important to note that drain 44 is of greater size than the inlets 54 of upright tubes 32. As a consequence, solid materials which cannot flow through the tubes 32 are collected in header 44 and ultimately discharge therefrom via orifice 44 into the hot water collection basin 20 therebelow automatically.

Since in most instances, a number of heat exchange structures 30 are provided for each tower 10!, it has been found desirable to provide a common vent line 46 intercommunicating all of the reverse flow manifold headers 36 through the medium of vent pipes 48 joining a respective header 36 to the interior of vent line 46. Interposed in each line 48 between vent line 46 and a header 36 is a valve 50 so that one or more of the reverse flow manifold headers 36 may be selectively disconnected from the liquid circuit.

Horizontal header feed lines 31 are joined to respective inlets 52 of hot water headers 34 and it is to be seen that each inlet 52 is of greater size than drain orifices 44 although the latter are larger than the inlet openings 54 of each of the upright finned tubes 32. The actual dimensions of the pipes, orifices and weir passages logically are constructed to meet specific applications in accordance with the total water flow rate. Typically, and based on the components as shown in the drawings and the relative sizes thereof as depicted, the inlet pipe 31 could be about 8 in. 1D. thus causing inlet 52 to be of the same size. Orifice 44 would then be about 2 in., the inlet openings of tubes 54 would be approximately 0.9 to l in., and the overflow opening 40 would be about 3 in. vertical height and 12 ft. long as defined by the upright weir plate which terminates 4 in. above the floor plane in which orifice 42 is located.

A by-pass drain line 56 connected to each horizontal feed line 31 is located to drain into the upwardly opening hot water distribution basin therebelow and as schematically illustrated in FIG. 2, a manually positionable valve 58 is located in each by-pass drain line 56 for controlling gravitation of hot water from line 31 into basin 20.

In operation, assuming initially that the tubes 32 and 37 of a respective heat exchange structure 30 are empty, hot water supplied via line 26, riser 28, horizontal manifold 29 and feed line 31 flows into header 34 and thence upwardly through finned tubes 32 into reverse flow manifold header 36 (FIG. 5). A portion of the hot water by-passes header 34 via line 56 depending upon the setting of control valve 58 while another proportion of the water gravitates into hot water distribution basin 20 through the drain orifice 44. Water collected in header 36 flows downwardly through tubes 37 into weir box 38 until the water overflows the upper edge thereof defining overflow opening 40. A small amount of the water drains from weir box 38 through orifice 42 but because of the relative sizes between the open upper end of the box 38 and the end drain opening 42, the water rises to a level where the lower ends of tubes 37 are immersed in such water thereby providing a liquid seal. As a consequence, the region of the header 46 above the liquid level 60 therein is normally at a pressure below atmosphere. The vent line 46 joining all of the reverse flow manifold headers 36 provides ventilation air from downstream parallel exchangers to prevent a premature partial siphon effect, thereby permitting proper and simultaneous siphon initiation during filling of the tubes 32 and 37 so that there is no tendency for the heat exchanger structures 36 to be devoid of hot water which could present a starved condition in the heat exchangers which are the most remote from the water supply line. Water flowing downwardly in the tubes 37 from header 36 recovers the siphon head so that continuous supply of hot water via lines 26, 28, 29 and 31 forces the liquid to flow uninterruptedly through the structure 30 along an upward path defined by finned tubes 32 and thence downwardly through finned tubes 37. As best shown in FIG. 2, the cooling air normally flows along a path 62 as indicated so that the coolest air first encounters the coldest liquid for most efficient cooling.

During start-up, a considerably greater than normal water flow rate initially occurs through the hot water by-pass line 56 under the control of valve 58 and through the hot water header orifice 44 until such time as the water reaches full flow through the finned tubes 32 and 37. When full flow occurs, normal ventilation air paths via unfilled exchangers are closed by the filling water streams, thereby permitting simultaneous siphon initiation of the parallel exchangers, producing an increment of pump head recovery which is equivalent to the exchanger height minus the friction and dynamic losses within the exchangers. This causes the pressure in the header feed line 31 to be reduced thus decreasing the pressure drop across the header orifice 44 and the by-pass line 56. As a result, less water is by-passed through line 56 direct to basin 20 after the siphon is initiated. The valve 58 should empirically be set at a position which permits proper start-up flow rates in the heat exchange structure 30 with only a desired amount of hot liquid by-passing the heat exchanger while continuing to provide a convenient and automatically operable flow path for the drain cycle of the apparatus as will be explained.

In the event of forced or operator initiated shut-down of the pumping system feeding line 26 thereby interrupting delivery of water to structure 30, the transient condition which occurs during deactivation of the pump system requires rapid draining of tubes 32 and 37 so that freezing of water within the heat exchanger finned tubes or any other components of the apparatus is precluded since malfunction or discontinuance of operation of the pump can occur during subfreezing weather conditons. As the supply pump is shut down and the water flow through line 31 is reduced, the level of liquid in weir box 38 falls to a sufficiently low level to permit the lowermost ends of finned tubes 37 to be opened to the atmosphere, (FIG. 6). At this point in time, the normal water flow direction is reversed, since this component is isolated as a siphon loop in which the lowest point is represented by hot water by-pass line 56. The water therefore flows back upwardly in finned tubes 37 and thence downwardly from the finned tubes 32 for gravitation into hot water distribution basin 20 via by-pass line 56 as well as orifice 44 in header 34. In this manner, the water is most expeditiously removed from the heat exchange structure 30 in a way which does not require the regulation of any specific controls or valves and occurs automatically upon deenergization of the pump within the water supply system. The orifices 42 and 44 and weir box 38 and header 34 respectively permit water in the lower portion of these units to also gravitate into hot water collection basin 20.

During normal operation of the apparatus, water supplied via lines 26, 28, 29 and 31 is divided into three different flow paths. The first is directly into hot water basin 20 via by-pass line 56 having partially opened valve 58 therein. The second path is through the drain orifice 44 of header 34. The third path is into the weir box 38 for overflow therefrom or gravitation through orifice 42. As previously noted, any suspended solid particles which would tend to clog up the finned tubes 32 and 37 are collected in header 34 and drain therefrom through orifice 44 since the latter is of greater size than the inlet openings 54 of the individual tubes 32.

In the schematic showing of FIG. 3, it can be seen that hot water to be cooled may be supplied outboard of the cooling tower 10 rather than inboard as illustrated in FIG. 1, without affecting operation of the hydraulic flow-segregation system. Although the header 134 is possibly of somewaht greater size to permit the supply line 131 to be joined thereto in underlying relationship to the weir box 138, this makes no discernable change in the operation of the equipment with the overflow from the open top of weir box 138 simply cascading down over line 131 into the underlying collection basin 20.

FIG. 4 of the drawings is a schematic illustration of the use of the present hydraulic flow-segregation system for a single pass heat exchanger wherein water to be cooled is delivered to the reverse flow manifold header 136 via supply conduit 132 (actually in the plane of finned tubes 137 rather than as shown for purposes of clarity as being to one side thereof) whereupon the water from header 136 flows downwardly into the weir box 138 for discharge therefrom in the same manner as previously described.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. In a multiple pass air cooled heat exchanger for cooling a fluid, the improvement comprising:

fluid conveying structure for bringing the fluid into thermal interchange relationship with cooling air and including separate intercommunicating sections for conveying the fluid to be cooled in first one direction and then another direction relative to said cooling air;

means for supplying fluid to be cooled to one of the sections for flow therethrough and thence through the other section; and

means communicating with both of the sections for effecting draining of fluid therefrom in response to a decrease in the supply of fluid to said one section.

2. A heat exchanger as set forth in claim 1 wherein said structure is provided with upright, side-by-side sections for receiving the fluid to be cooled and reverse flow manifold header means intercommunicating the sections.

3. A heat exchanger as set forth in claim 1 wherein said sections are disposed in upright positions for flow of the fluid along a first upright path and thence along a second upright path and the means for effecting draining of the sections is in communication with the lower portion of both of the sections.

4. A heat exchanger as set forth in claim 3 wherein said means for supplying fluid to the sections is connected to the lower portion of said one section for flow of fluid upwardly through said one section and thence downwardly through the other section.

5. A heat exchanger as set forth in claim 4 wherein said sections each include a series of upright tubes for parallel flow of fluid therethrough, sai means for supplying fluid to the sections being provided with a header in communication with the lower extremities of the tubes of said one section and means associated with said header for supplying fluid thereto and for permitting draining of said one section therethrough.

6. A heat exchanger as set forth in claim 4 wherein is provided an atmospheric weir box communicating with said other section at the lower end thereof and provided with said other section at the lower end thereof and provided with an overlfow opening and a drain orifice therebelow, said opening being larger than the drain orifice, the means for supplying fluid to the one section including a header communicating with the lower end of the one section and provided with a fluid inlet opening and a drain orifice therebelow, the fluid inlet opening being larger than the header drain orifice, and a fluid conveying line communicating with the fluid inlet of said header for supplying fluid to be cooled thereto.

7. A heat exchanger as set forth in claim 6 wherein is provided a by-pass drain line connected to said supply line and provided with a fluid flow control valve in operable association therewith for permitting selective variation of the flow of fluid from the supply line through said by-pass drain line.

8. A heat exchanger as set forth in claim 7 wherein is provided a distribution basin underlying the weir box,

said header and the by-pass line for receiving flow of fluid from the same.

9. A heat exchanger as set forth in claim 7 wherein the drain orifice of the header is of larger size than the inlet of said one section.

10. In an air cooled heat exchanger for cooling a fluid, the improvement comprising:

fluid conveying structure for bringing the fluid into thermal interchange relationship with cooling air and having an upper fluid inlet and a fluid outlet therebelow for gravitational fluid flow therefrom;

upright means communicating with said inlet of the structure for supplying fluid to be cooled thereto; and

means for effecting gravitational draining of the fluid from said structure and from the upright fluid supply means in response to a decrease in the supply of fluid to said upright means.

11. A heat exchanger as set forth in claim 10 wherein said means for effecting draining of the structure includes a weir unit at the outlet end of said structure and operable to permit gravitational draining of fluid from the structure and said weir unit in response to interruption of the supply of fluid to the structure.

12. A heat exchanger as set forth in claim 10 wherein said fluid supply means includes upright fluid conveying condiuit means extending alongside the structure in proximity thereto, a header joined to the conduit means at the lower end thereof, a fluid supply line coupled to the header and means operably associated with said line for controlling supply of fluid to the conduit means, said header being provided with orifice means in the lower portion thereof for gravitational draining of fluid from the conduit means and the header upon interruption of supply of fluid to the conduit means.

13. A heat exchanger as set forth in claim 12 wherein said means for effecting draining of the structure includes a weir box at the outlet end of said structure and having an overflow opening adjacent the level of said outlet end of the structure, said weir box being provided with an orifice in the lower portion thereof for gravitational draining of fluid from the weir box.

14. A heat exchanger as set forth in claim 13 wherein said overflow opening of the weir box is of greater size than said orifice therein.

15. A heat exchanger as set forth in claim 10, wherein is provided a distribution basin underlying the fluid conveying structure and said upright means for receiving flow of fluid thereform, a fill assembly underlying said distribution basin in disposition to receive fluid therefrom, and means for moving cooling air along separate paths through said structure and the fill assembly.

16. A heat exchanger as set forth in claim 15, wherein is provided a fluid supply line connected to said structure, a by-pass drain line joined to said supply line and located to direct fluid into said distribution basin and to permit draining of said upright means thereinto, and a fluid flow control valve in operable association with said drain line for permitting selective variation of the flow of fluid from the supply line through said by-pass drain line.

17. A heat exchanger as set forth in claim 15, wherein is provided means for effecting combination of the air streams moving through the structure and said fill assembly respectively prior to return thereof to the ambient atmosphere. 

1. In a multiple pass air cooled heat exchanger for cooling a fluid, the improvement comprising: fluid conveying structure for bringing the fluid into thermal interchange relationship with cooling air and including separate intercommunicating sections for conveying the fluid to be cooled in first one direction and then another direction relative to said cooling air; means for supplying fluid to be cooled to one of the sections for flow therethrough and thence through the other section; and means communicating with both of the sections for effecting draining of fluid therefrom in response to a decrease in the supply of fluid to said one section.
 2. A heat exchanger as set forth in claim 1 wherein said structure is provided with upright, side-by-side sections for receiving the fluid to be cooled and reverse flow manifold header means intercommunicating the sections.
 3. A heat exchanger as set forth in claim 1 wherein said sections are disposed in upright positions for flow of the fluid along a first upright path and thence along a second upright path and the means for effecting draining of the sections is in communication with the lower portion of both of the sections.
 4. A heat exchanger as set forth in claim 3 wherein said means for supplying fluid to the sections is connected to the lower portion of said one section for flow of fluid upwardly through said one section and thence downwardly through the other section.
 5. A heat exchanger as set forth in claim 4 wherein said sections each include a series of upright tubes for parallel flow of fluid therethrough, sai means for supplying fluid to the sections being provided with a header in communication with the lower extremities of the tubes of said one section and means associated with said header for supplying fluid thereto and for permitting draining of said one section therethrough.
 6. A heat exchanger as set forth in claim 4 wherein is provided an atmospheric weir box communicating with said other section at the lower end thereof and provided with an overlfow opening and a drain orifice therebelow, said opening being larger than the drain orifice, the means for supplying fluid to the one section including a header communicating with the lower end of the one section and provided with a fluid inlet opening and a drain orifice therebelow, the fluid inlet opening being larger than the header drain orifice, and a fluid conveying line communicating with the fluid inlet of said header for supplying fluid to be cooled thereto.
 7. A heat exchanger as set forth in claim 6 wherein is provided a by-pass drain line connected to said supply line and provided with a fluid flow control valve in operable association therewith for permitting selective variation of the flow of fluid from the supply line through said by-pass drain line.
 8. A heat exchanger as set forth in claim 7 wherein is provided a distribution basin underlying the weir box, said header and the by-pass line for receiving flow of fluid from the same.
 9. A heat exchanger as set forth in claim 7 wherein the drain orifice of the header is of larger size than the inlet of said one section.
 10. In an air cooled heat exchanger for cooling a fluid, the improvement comprising: fluid conveying structure for bringing the fluid into thermal interchange relationship with cooling air and having an upper fluid inlet and a fluid outlet therebelow for gravitational fluid flow therefrom; upright means communicating with said inlet of the structure for supplying fluid to be cooled thereto; and means for effecting gravitational draining of the fluid from said structure and from the upright fluid supply means in response to a decrease in the supply of fluid to said upright means.
 11. A heat exchanger as set forth in claim 10 wherein said means for effecting draining of the structure includes a weir unit at the outlet end of said structure and operable to permit gravitational draining of fluid from the structure and said weir unit in response to interruption of the supply of fluid to the structure.
 12. A heat exchanger as set forth in claim 10 wherein said fluid supply means includes upright fluid conveying conduit means extending alongside the structure in proximity thereto, a header joined to the conduit means at the lower end thereof, a fluid supply line coupled to the header and means operably associated with said line for controlling supply of fluid to the conduit means, said header being provided with orifice means in the lower portion thereof for gravitational draining of fluid from the conduit means and the header upon interruption of supply of fluid to the conduit means.
 13. A heat exchanger as set forth in claim 12 wherein said means for effecting draining of the structure includes a weir box at the outlet end of said structure and having an overflow opening adjacent the level of said outlet end of the structure, said weir box being provided with an orifice in the lower portion thereof for gravitational draining of fluid from the weir box.
 14. A heat exchanger as set forth in claim 13 wherein said overflow opening of the weir box is of greater size than said orifice therein.
 15. A heat exchanger as set forth in claim 10, wherein is provided a distribution basin underlying the fluid conveying structure and said upright means for receiving flow of fluid therefrom, a fill assembly underlying said distribution basin in disposition to receive fluid therefrom, and means for moving cooling air along separate paths through said structure and the fill assembly.
 16. A heat exchanger as set forth in claim 15, wherein is provided a fluid supply line connected to said structure, a by-pass drain line joined to said supply line and located to direct fluid into said distribution basin and to permit draining of said upright means thereinto, and a fluid flow control valve in operable association with said drain line for permitting selective variation of the flow of fluid from the supply line through said by-pass drain line.
 17. A heat exchanger as set forth in claim 15, wherein is provided means for effecting combination of the air streams moving through the structure and said fill assembly respectively prior to return thereof to the ambient atmosphere. 